Enterprise storage systems typically containmultiple storage tiers, each having its own performance, reliability, and recoverability. The primary motivation for this multi-tier organization is cost, as storage tier costs vary considerably. In this paper, we describe a file system called TierFS that stores files at multiple storage tiers while providing high recoverability at all tiers. To achieve this goal, TierFS uses several novel techniques that leverage coupling between multiple tiers to reduce data loss, take consistent snapshots across tiers, provide continuous data protection, and improve recovery time. We evaluate TierFS with analytical models, showing that TierFS can provide better recoverability than a conventional design of similar cost.